1. Field of the Invention
The present invention relates to radio communication systems, and particularly to a radio communication system for controlling radio-signal communications in a radio local area network (LAN).
2. Description of the Related Art
Radio LANs, which use electromagnetic waves for communications, have been widely spread in recent years, and the market thereof has been expanding. Since radio LANs do not use cables, cable-placing time and labor are unnecessary, and terminals can be installed at any places and moved. The needs for radio LANs have been increasing not only in companies but also in homes, and further market growth is expected.
Radio LANs are standardized in the Institute of Electrical and Electronics Engineers (IEEE) 802.11. It includes three extended specifications, IEEE 802.11a, IEEE 802.11b, and IEEE 802.11g.
In IEEE 802.11a, communications are performed at a rate of up to 54 Mbps, and the 5-GHz frequency band is used. Currently, IEEE 802.11b is used most frequently, has a maximum rate of 11 Mbps, and uses the 2.4-GHz frequency band.
IEEE 802.11g uses the same 2.4-GHz frequency band as IEEE 802.11b, but can provide about-five-times higher-speed communications at a rate of about 54 Mbps. In addition, unlike IEEE 802.11a, IEEE 802.11g is compatible with IEEE 802.11b. Antennas and radio-related sections can be designed in the same way as in IEEE 802.11b. With these reasons, IEEE 802.11g attracts attention as a future main standard, and products conforming thereto have been developed.
In radio LANs, an access point and a mobile terminal are connected by radio for communications. An “access point” in radio LANs means a relay unit (radio station) connected to the LANs.
An access point sends a beacon signal to indicate the existence thereof, and a mobile terminal searches for a beacon signal. In a radio-LAN connection conforming to IEEE 802.11g, a beacon signal needs to have a specified frequency in each of channels (CHs) 1 to 13, and each access point sends a beacon signal belonging to one specified channel to the air.
A mobile terminal sequentially searches channels 1 to 13 for a beacon signal. When it receives a beacon signal having the frequency of a certain channel, it locks at the channel, stops the searching (scanning) operation, and connects to the corresponding access point.
FIG. 11 shows a connection operation in a radio LAN. An access point 101 sends a CH1 beacon signal. A mobile terminal 114 moves its position and enters the radio area of the access point 101 while it searches channels 1 to 13 for a beacon signal.
When the mobile terminal 114 receives (captures) the CH1 beacon signal to detect the access point 101, it stops the scanning operation, and connects by radio to the access point 101, which is the transmission source of the CH1 beacon signal. Assuming that the access point 101 has already been connected by radio to mobile terminals 111 to 113, it now is connected to a total of four mobile terminals 111 to 114.
A conventional technology for radio-LAN connections has been disclosed in which a mobile terminal selects an access point to be connected (for example, in paragraphs [0025] and [0026] and FIG. 1 of Japanese Unexamined Patent Application Publication No. 2002-51368).
When a plurality of access points which send beacon signals in different channels (frequencies) are installed, there may be a radio area where beacon signals of different channels exit depending on the installation condition. In that case, if mobile terminals are located at the radio area, many mobile terminals may connect to the same access point, increasing the processing load of the access point.
FIG. 12 to FIG. 14 show this drawback. In FIG. 12, there are two access points 101 and 102. The access point 101 sends a CH1 beacon signal, and the access point 102 sends a CH2 beacon signal. In the CH1 radio area B1 of the access point 101 and the CH2 radio area B2 of the access point 102, there is a radio area B3 where the radio area B1 and the radio area B2 overlap.
It is assumed that mobile terminals 111 to 114 have already connected to the access point 101 by the channel 1, and mobile terminal 115 and 116 have already connected to the access point 102 by the channel 2. Mobile terminals 117 and 118 located in the radio area B3, the overlapping area, can connect to either of the access points 101 and 102.
When the mobile terminals 117 and 118 connect to the access point 102, as shown in FIG. 13, the access points 101 and 102 have an equal processing load. When the access points 101 and 102 send data to the mobile terminals 111 to 118 at 2 Mbps, for example, the access points 101 and 102 send 8-Mbps data in an equal manner.
When the mobile terminals 117 and 118 connect to the access point 101, as shown in FIG. 14, the access point 101 has a higher processing load. When the access points 101 and 102 send data to the mobile terminals 111 to 118 at 2 Mbps, for example, the access points 101 sends 12-Mbps data and the access point 102 sends 4-Mbps data.
In this way, depending on the connections of mobile terminals, it is conventionally possible that one access point has a high processing load while the other access point has a low processing load, which causes an inefficient operation.
In the above-described conventional technology (Japanese Unexamined Patent Application Publication No. 2002-51368), control is made such that a mobile terminal detects the traffic volumes of data communication at access points to select an access point having a low traffic volume, so as to make the processing loads of access points almost uniform. Since mobile terminals have an access-point selection function in such control, it is necessary to add the function to all mobile terminals (notebook computers, or the like) commercially sold, which is not necessarily an optimum solution.